I vaguely seem to remember one in it's late 30's but tbh the bigger the horse the shorter the life span (generally) something to do with relative heart sizes and is why ponies go on forever!
There's one at the end of our road, lives out 24/7 (well rugged) and he's about 30 according to the dentist/vet and looks smashing, all nice and shiny when he's been groomed and not been rolling in the mud!
I have been checking out my ex racers line and he has alot of relatives who were in there 30's or very late 20's and bearing in mind they were all racers this is pretty good i think. His grandad was rainbow quest who only died a couple of years ago (quite remeber how old but he was getting on) and another of his relatives on his sires side was the oldest working sire on record, again into his 30's!!